Discharge lamp having cavity electrodes

ABSTRACT

A high-intensity electric discharge lamp having cavity electrodes operating in the hollow cathode mode. Penetration of the arc terminus into the electrode cavity is assisted by vapor breathing which injects plasma into the cavity during AC reignition after current zero. Breathing is favored by a cavity which is gastight and which has a depth not substantially greater than the terminus penetration. Relatively low temperatures deep within the cavity are avoided by providing enhanced thermal coupling between the forward end of the cavity member and the radiation shield surrounding it, and also by thermal insulation between the sides of the cavity member and the shield. Emission material located within the lower portion of the cavity favors deep terminus penetration. Projection of the radiation shield beyond the cavity member toward the other electrode is avoided.

United States Patent [72] Inventor John E. White Cleveland Heights, Ohio [21] Appl. No. 40,134 [22] Filed May 25,1970

Continuation-impart of application, Ser. No. 769,037, Oct. 21, 1968 [45] Patented Nov. 9, 1971 [73] Assignee General Electric Company [54] DISCHARGE LAMP HAVING CAVITY ELECTRODES 16 Claims, 7 Drawing Figs.

[52] U.S.Cl 313/211, 313/184, 313/209, 313/229, 313/346 [51] Int. Cl I-I0lj 61/06 [50] Field of Search 313/184, 185,207, 209, 211, 213, 229, 346

[56] References Cited UNITED STATES PATENTS 3,029,359 4/1962 White 313/185 Primary Examiner-Raymond F. Hossfeld Attorneys-Ernest W. Legree, Henry P. 'Iruesdell, Frank 1..

Neuhauser and Oscar B. Waddell ABSTRACT: A high-intensity electric discharge lamp having cavity electrodes operating in the hollow cathode mode. Penetration of the arc terminus into the electrode cavity is assisted by vapor breathing which injects plasma into the cavity during AC reignition after current zero. Breathing is favored by a cavity which is gastight and which has a depth not substantially greater than the terminus penetration. Relatively low temperatures deep within the cavity are avoided by providing enhanced thermal coupling between the forward end of the cavity member and the radiation shield surrounding it, and also by thermal insulation between the sides of the cavity member and the shield. Emission material located within the lower portion of the cavity favors deep terminus penetration. Projection of the radiation shield beyond the cavity member toward the other electrode is avoided.

PATENTEDN 9"}?! 3519.699

SHEET 1 BF 2 25 Z j ITWVEETWTOT'I John E. White His A t tovneg DISCHARGE LAMP HAVING CAVITY ELECTRODES BACKGROUND OF THE INVENTION This application is a continuation-in-part of my copending application, Ser. No. 769,037, filed Oct. 21, 1968, entitled High Intensity Metal Vapor Lamp Having Cavity Electrodes, assigned to the same assignee and now abandoned. The invention relates to thermionic-emitting cavity electrodes for use in high-intensity lamps, particularly metal vapor lamps.

In my US. Pat. No. 3,029,359 I have described and claimed a thermionic hollow cathode which operates well with little envelope blackening at currents up to 75 amperes. That cathode comprises a hollow cuplike body of tungsten open towards the front, with a tungsten coil lining the cavity walls and having emission material coated thereon and lodged in the interstices between turns.

In my copending application Ser. No. 769,038, filed Oct. 21, 1968, entitled High Current Thermionic Hollow Cathode Lamp, now U.S. Pat. No. 3,558,964, and similarly assigned, I have described and claimed a thermionically emitting hollow cathode particularly suitable for very high-current xenonfilled lamps and capable of supporting currents up to 400 amperes. An important feature of that cathode is a dead space within the cavity into which gas can expand during the AC cycle to reduce vapor breathing, that is gas flow in and out of the open end of the hollow cathode.

The object of my present invention is to provide electrodes operating in the hollow cathode mode at lower currents in the range of 0.5 to amperes. This is the range of currents commonly encountered in the popular sizes of high-intensity or high-pressure metal vapor lamps such as mercury, sodium and metal halide lamps for commercial, industrial and streetlighting applications.

My present invention is of particular interest in connection with mercury metal halide lamps which achieve a substantial improvement in color rendition and efficiency relative to conventional mercury arc lamps by adding one or more vaporizable metal halides to the mercury filling. Such lamps are described and claimed in U.S. Pat. No. 3,234,421 to Gilbert I-l. Reiling, Metallic Halide Electric Discharge Lamps, issued Feb. 8, 1966. A preferred filling comprises mercury, sodium iodide, thallium iodide and indium iodide and achieves an efficiency of 80 to 90 lumens per watt with a white to nearwhite color.

Mercury metal halide lamps suffer from shorter life and poorer lumen maintenance by comparison with conventional mercury lamps. The problem is related to chemical reactions between the electron-emitting materials used on the electrodes and the metal halide constituents of the ionizable filling. In the conventional mercury lamp, alkaline earth oxides such as BaO are used which are very efficient electron emitters. In the mercury metal halide lamp, less efficient Th0 is used in order to avoid chemical reactions, but efficiency, life and maintenance are not as good. Therefore an efficient hollow cathode having long life and good maintenance without requiring an alkaline earth oxide electron-emitting material would be particularly useful in mercury metal halide lamps.

SUMMARY OF THE INVENTION Electron emission in lamps may be characterized in three distinct modes: The Fowler-Nordheim, the Schottky spot mode, and the diffuse mode. In the Fowler-Nordheim mode, emission is characterized by mobility of the arc terminus and a high voltage peak at current zero in AC lamps. The Schottky spot mode is that most commonly encountered in high-intensity lamps of the metal vapor type wherein the arc attaches to a spot on the electrode which is at a much higher temperature than the rest of the structure. In the diffuse mode, electron emission takes place from a large area and is supplemented little by ion collection. The diffuse mode is seldom observed at high pressure.

The benefits of diffuse mode operation, particularly when combined with a hollow electrode geometry, are low electrode losses, and entrapment within the electrode cavity of metallic material released from the electrodes. The first factor means higher efficiency, and the second means reduced envelope blackening. My invention provides an electrode structure which achieves the diffuse hollow cathode mode of operation in the relatively high-pressure metal vapor filling of high-intensity lamps.

In accordance with my invention, l have found that penetration of the arc terminus into the electrode cavity is assisted by vapor breathing which injects plasma into the cavity during AC reignition after current zero, and that such breathing is very desirable in high-pressure low-current lamps. Breathing is favored by a cavity which is gastight and which has a depth not substantially greater than the tenninus penetration. High temperatures deep within the cavity are desirable and are achieved by providing enhanced thermal coupling between the forward end of the cavity member and the cooler radiation shield surrounding it, and also by thermal insulation between the sides of the cavity member and the cooler shield. It is desirable to locate emission material within the lower portion of the cavity because it favors deep tenninus penetration. Projection of the radiation shield beyond the cavity member toward the other electrode is avoided because such projection would favor formation of a spot mode terminus on the shield.

DESCRIPTION OF DRAWINGS FIG. 1 is a front elevation view of a high-intensity mercury metal vapor arc lamp provided with cavity electrodes embodying the invention.

FIGS. 2a and b are a partly sectioned front elevation and an end view of a cavity electrode design embodying the invention.

FIG. 3 is a sectioned front elevation of another design of cavity electrode embodying my invention.

FIG. 4 is a sectioned front elevation of yet another design of cavity electrode embodying my invention.

FIG. 5 illustrates conditions at the cavity electrode during reignition after current zero.

FIG. 6 illustrates conditions at the cavity electrode during current maximum.

GENERALIZED DISCLOSURE l. Mobility Limitations At low currents, the problem of inducing cavity moding is much more severe than at high currents. This is readily appreciated from the following simplified set of assumptions. Consider a cylindrical (unshielded) electrode design having a fixed cavity axial depth and operating under fixed pressure conditions. Let the cavity diameter be varied as needed with the current, and let the cavity depth be chosen such that at all currents of interest, the arc terminus will burn to the full depth of the cavity in a diffuse cavity mode. Under these conditions the power input to the electrode will be proportional to the current. To a first approximation, since the power dissipated from the electrode at a given temperature is proportional to the area of the electrode, a constant temperature can be achieved by varying the diameter d of the cavity proportionally to the current I:

The area A of the cavity opening into which the current must flow is proportional to the square of the diameter:

A=k,a' (2) Combining (1) and (2) to eliminate d:

A=k F (3),

and rearranging I/A=k /l (4).

Thus to a first approximation, the current density !/.4 will be inversely proportional to the current. If the electron-emissive properties of a cavity are such as to require a certain cavity temperature, a high-current application will lead to a small required current density in the cavity while a low-current application will lead to a high-current density. For a given electron mobility, then, it is expected that large currents can easily ELECTRODE CONSTRUCTIONS In lamps embodying my invention, the foregoing requirements have been met by relatively small compact electrodes. One type comprises two separate concentric coils of refractory metal wire, the inner coil being hollow and open in the direction of the arc. Suitable refractory metals are tungsten, tantalum or rhenium or alloys, tungsten being preferred. A heat conservation means is provided such as a radiation shield interposed between inner and outer coils.

In another type three concentric coils are used wherein the intermediate coil takes over the function of the radiation shield. The inner coil projects beyond the outer coil in order to assure a minimum voltage to the cavity region. An electronemitting material is provided in the cathode, suitably thoria for lamps containing sodium iodide in addition to mercury. The thoria is applied as a pellet or tablet deep within the cavity to serve as a reservoir. It is desirable for the thoria to contain a small amount of activator to promote the release of free thorium or a more readily volatilizcd compound of thorium. Nitrocellulose or other carbon-containing compound may be used for the activator. Tungsten carbide WC is preferred, a suitable proportion being 30 percent WC by weight mixed in a dry pellet ofThO,

In the initial arcing of the lamp, a small amount of free Th or ThC is generated which travels to the cavity wall and there produces sufficient lowering of the work function to cause the arc terminus to burn in the hollow cathode mode within the cavity in proximity to the pellet. The ion bombardment and temperature during operating cause sufficient thoria or its products to be dispensed to the cavity to make up for losses out the cavity mouth. I have found that a primer or thin layer of thoria coating the inner surface of the cavity will encourage the initial penetration of the arc into the cavity. However this is not essential because ion bombardment of the pellet during operation will provide thoria and its disproportionating products along the walls of the cavity in any event. Locating the pellet at the bottom of the cavity results in having the lowest work function at the bottom which fosters deep burnmg.

Alternatively, it is possible to use a priming material admixed with the thoria, instead of the activator. This material must have a low work function, and dispense from the emission pellet during lamp processing. The priming material thus provides a low cavity work function during the period before ion bombardment has provided adequate dispensing of thoria to the cavity walls. Dy,0 and La O have been found to be satisfactory priming materials. They have been mixed with the thoria in 1 to 5 mol percent of the thoria quantity. Under suitable circumstances, either of the above materials or Y O may be substituted for the thoria itself as the electron emitting material.

Cavity diameter will of course depend upon the current rating of the lamp and the effectiveness of the emission material in lowering the work function. Using thoria as the emission material, the range of useful cavity diameter extends from about 20 to I00 mils for the common sizes of high-intensity metal vapor lamps ranging from 50 watts to 1,000 watts and having currents ranging from 0.6 to 8 arnperes.

Yet another type of hollow cathode according to the invention may be made through fabrication techniques using powder metallurgy molding or machining. The inner coil in particular lends itself to replacement by a pressed tungsten insert or cavity member having similar thermal and thermionic properties. The cavity member may consist of tungsten alone at the top or forward end, and tungsten admixed with electron-emitting material such as thorium oxide at the lower or inner end. By this arrangement, a low work function region is achieved deep within the cavity which promotes deep penetration of the are terminus. The cavity member is surrounded by a radiation shield which contacts the cavity member most firmly at the upper or forward end. A space or absence of pressure contact between the cylindrical sides and the circular bottom of the cavity member and the cooler shield surrounding it achieves thermal insulation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the drawing and FIG. 1 in particular, my invention is embodied in a high pressure mercury metal halide vapor lamp 1 comprising an outer vitreous envelope or jacket 2 of bulbous shape with a tubular neck portion 3. A conventional screw base 4 on the end of the neck is connected to inleads 5,6 sealed through a reentrant stem 7. An inner quartz arc tube 8 is supported within the outer jacket by a divided mount comprising support rods 9,10 and clamping bands 11,12. Sealed within the ends of the arc tube are a pair of cavity electrodes 13,13 embodying the invention. The electrodes are connected to inleads 5,6 by conductors 14,15.

The are tube contains a quantity of mercury most of which is entirely vaporized during operation of the lamp and which at such time exerts a pressure in the range of 1 to 15 atmospheres. A quantity of sodium iodide is provided appreciably in excess of that vaporized at the operating temperature of the arc tube which is not less than 500 C. at any place. Thallium iodide and indium iodide may also be provided, in addition to sodium iodide, for improved color or efficiency. The coolest region of the arc tube during operation are the ends and to insure that they do not drop below 500 C., a heatabsorptive coating indicated by speckling may be applied to the ends and to the adjacent portions of the pinch seals. Also as a heat conservation measure in the 400-watt size of lamp illustrated, the interenvelope space is evacuated. In the larger sizes of lamp such as the 1,000-watt size, such evacuation is not necessary.

Wound ElectrodeTwo Layer An electrode 13a is shown to a much larger scale in FIGS. 2a and b. Each electrode comprises a tungsten rod or shank 17 on the end of which are wound the first few (4) turns 18 of an outer coil of tungsten wire, shown partly cut away. The remaining (4) turns 19 projecting forward of the shank are wound to a larger diameter. Within the larger turns 19 is located an inner coil 20 likewise of tungsten wire, the inner coil extending beyond the outer coil in the direction of the opposite electrode by more than about one-half wire diameter. A heat radiation shield of sheet tungsten 21 is located between inner and outer coils in the overlapped portion. The sheet is preferably longitudinally crimped as best seen in FIG. 2b in order to reduce the area of contact between the turns of the coil and the sheet so as to minimize direct heat conduction between the parts. The inner coil should not extend beyond the outer coil by more than a few turns in order to avoid excessive radiation loss. In FIG. 2a, the projection is about one wire diameter, indicated 20a. A pellet or compressed tablet 22 of thoria is pressed into the bottom of the cavity and serves as a reservoir to replenish any thoria lost during operation. Ion bombardment of the pellet during operation vaporizes thoria and its disproportionating products which condense on the walls of the cavity. By locating the pellet or reservoir of emission material at the bottom of the cavity, the lowest work function is obtained deep within the cavity and this fosters deep burning of the arc. The thoria pellet preferably contains an activator to foster early vaporization and deposition of disproportionating products on the cavity walls as previously described.

Too large a cavity diameter works against the heat conservation requirement while too small a diameter works against adequate conduction passage. In the case of a 400-watt lamp operating at a current of 3 to 4 amperes, usually about 3.5 amperes, the cavity diameter should preferably be within the range from 30 to 40 mils. In the preferred embodiment illustrated in FIGS. 20 and 2b, inner coil 20 consists of l2-mil tungsten wire, outer coil l8,l9 consists of l5-mil tungsten wire, radiation shield M is l-mil thick tungsten sheet, and

ln assembling grid electrode 7 shown in FIG. 4a with the anode electrode shown in FIG. 5, projections 14' are fitted in guide openings 14 is above described to bring grid electrode 7 close to the insulating substrate 2 thus providing an assembly as shown in FIG. 6. Filament 6 is then mounted to extend in front of the assembly and lead wires are connected to respective electrodes. The assembly is then sealed in the glass envelope 9 to complete the fluorescent tube shown in FIG. 3.

As above described the function of a control grid that controls the divergence and flow of electrons and that of a screen grid electrode that collects the substances evaporated from fluorescent anode segments and secondary electrons emanated therefrom are provided by a single grid electrode. As a result, the number of electrodes is decreased by one thus providing an inexpensive fluorescent display tube wherein electrodes can be assembled readily.

The grid electrode employed in this invention has a window of a configuration of a pattern comprised by all fluorescent segments on an insulating substrate and a mesh is formed in the window. Furthermore, different from a prior screen grid, as there is no bridge in the window corresponding to and aligning with the insulating bridges between segments on the insulating substrate, it is not necessary to take care to align the window with the insulating bridge on the insulating substrate, thus rendering easy assembling. This construction also eliminates the provision of a control grid between the screen grid and the filament which was essential to the prior construction so that it becomes possible to dispose the filament closer to the anode electrode thus flattening and miniaturizing the display tube. The cutoff characteristics of the novel display tube has been improved about 30 percent over the prior fluorescent display tube.

What is claimed is:

1. A fluorescent display tube comprising an evacuated sealed envelope, an insulating substrate within said envelope, an anode electrode including a plurality of fluorescent anode segments adapted to form characters, said anode segments being mounted on said insulating substrate and insulated from each other, a single grid electrode disposed close to said anode segments, and a cathode filament extending in front of said grid electrode, said grid electrode being provided with a window common to the region of a pattern formed by said plurality of fluorescent anode segments, said window being provided with an electroconductive mesh.

2. The fluorescent display tube according to claim I wherein said grid electrode comprises a metal plate provided with a window of a configuration corresponding to a pattern formed by all segments of a character to be displayed and a metal mesh provided for said window and wherein said anode comprises a plurality of fluorescent anode segments embedded in said insulating substrate and insulated from each other, said anode segments being arranged to be combined to display a desired character or digit and wherein said grid electrode and said anode electrode are assembled together.

3. The fluorescent display tube according to claim I wherein said grid electrode and said substrate are connected together by means of interfitting projections and openings.

11. A high-intensity high-pressure lamp comprising a vitre ous envelope containing an ionizable medium and having a pair of electrodes sealed therein at least one of which is a thermionic self-heating cavity electrode operating in the hollow cathode mode, said one electrode comprising an inner cavity member of refractory metal and an outer shielding member surrounding it, said inner member having a cavity therein which is substantially gastight but open at the front end facing the arc, said cavity having a depth not substantially greater than the penetration of the arc terminus therein, said inner member having enhanced thermal coupling to said outer member at the front end, and electron-emitting material located within the lower portion of said inner member.

12. A lamp as in claim 11 wherein no part of said outer shielding member projects beyond said inner cavity member in the direction of the other electrode.

13 A lamp as in claim 11 wherein said inner and outer members are made of molded tungsten powder.

14. A lamp as in claim 11 wherein both inner and outer members are made of molded tungsten powder, said inner member consisting of substantially pure tungsten powder at the front end and of tungsten powder admixed with electronemitting material in the lower portion contained within said outer shielding member.

15. A lamp as in claim 14 wherein said inner member is in the form of a hollow cylinder and said outer member is in the form of a cup into which the inner member is fitted.

16. A lamp as in claim 15 wherein the outer diameter of said hollow cylinder tapers inwardly from front to rear relative to the diameter of the opening in said cup whereby pressure contact and enhanced thermal coupling occurs with the front only of said cup. 

1. A high intensity metal vapor lamp comprising a vitreous envelope containing metal vapors and having a pair of electrodes sealed therein at least one of which is a thermionic self-heating cavity electrode operating in the hollow cathode mode, said one electrode comprising an inner hollow coil of tungsten mounted on a metal shank and open at the front end facing the arc, an outer coil of tungsten wire concentric with said inner coil, a heat radiation shield between inner and outer coils, the inner coil projecting forward beyond the outer coil, the depth of the cavity in said inner coil being greater than its diameter, and a reservoir of emission material in the bottom of the cavity.
 2. A lamp as in claim 1 wherein the diameter of the cavity is in the range of 20 to 100 mils.
 3. A lamp as claim 1 wherein the heat radiation shield is a sheet of tungsten foil.
 4. A lamp as in claim 1 wherein the heat radiation shield is an intermediate winding of tungsten wire between inner and outer tungsten coils.
 5. A high-intensity metal vapor lamp comprising a vitreous envelope containing mercury and metal halide and having a pair of electrodes sealed therein at least one of which is a thermionic self-heating cavity electrode operating in tee hollow cathode mode, said one electrode comprising an inner hollow coil of tungsten wire mounted on a metal shank and open at the front end facing the arc, an outer coil of tungsten wire concentric with said inner coil, a heat radiation shield between inner and outer coils, the inner coil projecting forward beyond the outer coil, the depth of the cavity in said inner coil being greater than its diameter, and a reservoir of thoria in the bottom of the cavity.
 6. A lamp as in claim 5 wherein the diameter of the cavity is in the range of 20 to 100 mils.
 7. A lamp as in claim 5 wherein the heat radiation shield is a sheet of tungsten foil.
 8. A lamp as in claim 5 wherein the heat radiation shield is an intermediate winding of tungsten wire between inner and outer tungsten coils.
 9. A lamp as in claim 5 wherein the metal halide includes sodium iodide.
 10. A lamp as in claim 5 for operation at a current of 3 to 4 amperes wherein the metal halide includes sodium iodide and the cavity diameter is in the range of 20 to 40 mils.
 11. A high-intensity high-pressure lamp comprising a vitreous envelope containing an ionizable medium and having a pair of electrodes sealed therein at least one of which is a thermionic self-heating cavity electrode operating in the hollow cathode mode, said one electrode comprising an inner cavity member of refractory metal and an outer shielding member surrounding it, said inner member having a cavity therein which is substantially gastight but open at the front end facing the arc, said cavity having a depth not substantially greater than the penetration of the arc terminus therein, said inner member having enhanced thermal coupling to said outer member at the front end, and electron-emitting material located within the lower portion of said inner member.
 12. A lamp as in claim 11 wherein no pArt of said outer shielding member projects beyond said inner cavity member in the direction of the other electrode.
 13. A lamp as in claim 11 wherein said inner and outer members are made of molded tungsten powder.
 14. A lamp as in claim 11 wherein both inner and outer members are made of molded tungsten powder, said inner member consisting of substantially pure tungsten powder at the front end and of tungsten powder admixed with electron-emitting material in the lower portion contained within said outer shielding member.
 15. A lamp as in claim 14 wherein said inner member is in the form of a hollow cylinder and said outer member is in the form of a cup into which the inner member is fitted.
 16. A lamp as in claim 15 wherein the outer diameter of said hollow cylinder tapers inwardly from front to rear relative to the diameter of the opening in said cup whereby pressure contact and enhanced thermal coupling occurs with the front only of said cup. 